In recent years, the information wave has lead to increasing use of broadband circuits that are capable of communicating large volumes of information at high speed. Further, the devices used for transmitting information through these broadband circuits include transmission devices such as router devices and WDM (Wavelength Division Multiplexing) devices. A plurality of signal processing boards, which combine an arithmetic element such as an LSI and a storage element such as memory and the like, are installed in these transmission devices and maintain the interconnection between each of the circuits.
Each signal processing board is composed of a circuit in which the arithmetic element and the storage element and the like are connected with electrical wiring, but in recent years, as the amount of information requiring processing has increased, each board has been required to transmit information at an extremely high throughput. However, as the speed of information transmission has increased, problems such as the generation of crosstalk and high-frequency noise, and deterioration in the electrical signals have become apparent. As a result, the electrical wiring has become a bottleneck, and achieving further improvements in the throughput of the signal processing boards is proving difficult. Further, similar problems are also becoming apparent even in super computers and large-scale servers and the like.
On the other hand, optical communication technology that uses optical carrier waves to transport data has also been developed, and in recent years, optical waveguides are becoming increasingly widespread as a means of guiding an optical carrier wave from one point to another point. This optical waveguide has a linear core portion, and a cladding portion that is provided so as to cover the periphery of the core portion. The core portion is formed from a material that is substantially transparent to the light of the optical carrier wave, and the cladding portion is formed from a material having a lower refractive index than the core portion.
In an optical waveguide, the light introduced from one end of the core portion is transmitted (transported) to the other end of the core portion while reflecting off the interface with the cladding portion. A light emitting element such as a semiconductor layer is disposed at the input side of the optical waveguide, and a light receiving element such as a photodiode is disposed at the output side. The light input from the light emitting element is transmitted through the optical waveguide and is received by the light receiving element, and communication is achieved based on the blinking pattern or intensity pattern of the received light.
It is anticipated that by replacing the electrical wiring in a signal processing board with this type of optical waveguide, the aforementioned problems associated with electrical wiring can be resolved, enabling further increases in the throughput of the signal processing board.
For example, Patent Document 1 proposes a signal processing board provided with a plurality of optical waveguides, and a light emitting element and a light receiving element corresponding with each of those optical waveguides. In this signal processing board, the plurality of optical waveguides are formed in the signal processing board by disposing a cladding portion between adjacent core portions so as to contact both core portions.
However, in a signal processing board of this structure, as the packing density is increased and the board is reduced in size, the distance between adjacent core portions narrows, and as a result, crosstalk problems can occur as leaked signals from adjacent core portions enter the core portions.